Papers by Author: Hartmut Bracht

Abstract: The contributions of vacancies and self-interstitials to silicon (Si) self-diffusion are a matter of debate since many years. These native defects are involved in dopant diffusion and the formation of defect clusters and thus influence many processes that take place during Si single crystal growth and the fabrication of silicon based electronic devices. Considering their relevance it is remarkable that present data about the properties of native point defects in Si are still limited and controversy. This work reports recent results on the properties of native point defects in silicon deduced from self-diffusion experiments below 850°C. The temperature dependence of silicon self-diffusion is accurately described by contributions due to vacancies and self-interstitials assuming temperature dependent vacancy properties. The concept of vacancies whose thermodynamic properties change with temperature solves the inconsistency between self-and dopant diffusion in Si but further experiments are required to verify this concept and to prove its relevance for other material systems.

Abstract: We report experiments on the diffusion of n-type dopants in isotopically controlled Ge multilayer structures doped with carbon. The diffusion profiles reveal a strong aggregation of the dopants within the carbon-doped layers and a retarded penetration depth compared to dopant diffusion in high purity natural Ge. Dopant aggregation and diffusion retardation is strongest for Sb and similar for P and As. Successful modeling of the simultaneous self- and dopant diffusion is performed on the basis of the vacancy mechanism and additional reactions that take into account the formation of carbon-vacancy-dopant and dopant-vacancy complexes. The stability of these complexes is confirmed by density functional theory calculations. The overall consistency between experimental and theoretical results supports the stabilization of donor-vacancy complexes in Ge by the presence of carbon and the dopant deactivation via the formation of dopant-vacancy complexes. These results help to develop concepts to suppress the enhanced diffusion of n-type dopants and the donor deactivation in Ge. Both issues hamper the formation of ultra shallow donor profiles with high active dopant concentrations that are required for the fabrication of Ge-based n-type metal oxide semiconductor field effect transistors.

Abstract: Diffusion of alkaline-earth ions in mixed cation glasses of the composition xA2O•(3-x)MO•4SiO2 (x = 0.0, 0.1, 0.3, 0.4 and 1.0; A = Na, K; M = Ca, Ba) was investigated by means of the radiotracer diffusion technique below the respective glass transition temperatures. The mobility of alkaline-earth ions increases with the alkali content in all analyzed glass systems with no decrease in the diffusion activation energy, but a raise in the pre-exponential factor. A distinct dependency of the activation energy of the alkaline-earth ions on the type and content of the alkali ions in the glass is observed. Additional experiments with thin glass films derived by the sol-gel technique reveal an analogous diffusion behaviour. This demonstrates that the dynamic of alkaline-earth ions in mixed cation glasses does not depend on the way of glass preparation.

Abstract: Diffusion of oxygen (O) in amorphous silicon dioxide (SiO2) was investigated by means of Si3N4/natSinatO2/28Si18O2/28Si isotope heterostructures grown by thermal oxidation and plasma enhanced chemical vapour deposition. Diffusion experiments with and without a silicon nitride (Si3N4) cap, which serves as diffusion barrier for the gasses in the ambient, were performed. In particular, we determined the impact of the ambient gas, of the thickness of the isotopically enriched SiO2 layer, and of the annealing time and temperature on diffusion. Our results are compared with data given in the literature on oxygen and silicon diffusion in silica and are discussed in the framework of the experimental conditions established at the sample surface and at the buried 28Si18O2/28Si interface. Taking into account a point defect model that is predicted by recent atomistic simulations all experimental results can be explained consistently.